In such a communication system which realizes communications between modems connected through transmission lines, it is generally known that the modems in communication are synchronized at least during data transmission and reception between the sending terminal and receiving terminal connected to the respective modems and the modem in the receiving side is capable of equalizing the line characteristic.
In view of meeting such conditions, a training signal of a predetermined constant pattern containing at least a binary random data signal is transmitted from the modem in the sending side just before actual data transfer from, for example, the terminal in order to train the modem in the receiving side and thereby the receiving modem is placed in a condition matching said respective conditions just before transmission and reception of data between terminals.
However, such a conventional training method has a disadvantage in that the length of training signal is fixed and such a length must be sufficiently long so that the modem in the receiving side can be trained perfectly even, in case line characteristics are different.
Meanwhile, it has been proposed to realize communication between a modem in the center and a plurality of modems by connecting such a modem and the modems provided in a plurality of terminals.
In such a communication system the distance from the center modem to each terminal is different. Therefore, the conventional training method explained above requires longer training times before the start of data transmission from the terminals; and such times have not been employed for such a communication system because the time is determined in accordance with the transmission line having the worst line characteristic.
On the other hand, as a training method which assures very high speed training, the method disclosed in the specification of U.S. Pat. No. 3,962,637 is known.
FIGS. 1(a) and (b) show the conventional training method explained above. FIG. 1(a) indicates the block diagram of a modem, while FIG. 1(b) indicates the pattern of a training signal.
In FIG. 1(a), numeral 1 indicates an automatic gain controller (AGC), while 2 indicates a demodulating circuit, 3 indicates a roll-off filter, 4 denotes an equalizer, 5 indicates a phase controller, 6 indicates a discriminator, 7 deontes a code converter, 40 indicates a high speed pull-in circuit, 8 indicates a timing generator, and 9 indicates a carrier detector.
In FIG. 1(b), "tone" means a tone signal, "tim" means a timing signal, "imp" means an impulse, "dat" means a data and T0.about.T7 respectively indicate times.
A training means will be explained hereunder.
First, a training signal shown in FIG. 1(b) is sent from the sending side (now shown).
The modem in the receiving side causes, at first, the AGC 1 and pulling of phase of signal using the tone signal starting from the time T0 and obtains a detecting output CD from the detector 9.
The phase of the carrier signal is adjusted by the timing signal "tim" starting from the time T1 and the send timing is pulled by timing generator 8.
The impulse signal "imp" appearing between the time T3 and T6 of the training signal contains impulses i1 and i2 which are synchronized with the timing of data and provided with no-signal period in both sides.
This signal is supplied to the high speed pull-in circuit 40 from the phase controller 5. The high speed pull-in circuit 40 discriminates line characteristics by operating on components of distortion in the impulses i1 and i2; initializes the equalization coefficient of equalizer 4; fine-adjusts the phase controller 5; and moreover resets the code converter 7.
As explained above, the training of the receiving side continues until the time T6. The signal received during the next data "dat" period is gain-adjusted by the AGC 1, demodulated by the demodulator 2 and converted to the base band by the roll-off filter 3. The base band signal is subjected to elimination of line distortion in accordance with the line characteristic by the phase controller 5. After line distortion is eliminated, the base band signal is then subjected to data discrimination, decoded by the code converter 7 and supplied as a receiving data RD to the receiving terminal. According to this patent, a high speed pull-in, namely training is possible.
But, if the line characteristic is bad, this method cannot sufficiently equalize line distortion. Therefore, it is also proposed to add a binary random code in addition to this signal in such a communication system where various line characteristics exist.
However, when this method is introduced into the communication system where various line characteristics exist, the length of the training signal is fixed in order to discriminate the timing where the training signal and the data signal change in the receiving side in any of the conventional proposes. Moreover, the training is carried out using the training signal in the sufficient length for the modem to pull in the line having the worst line characteristic.
Therefore, even in the line having a good line characteristic, this method provides the disadvantage that, the time from starting the transmission of the training signal to being ready for sending the data signal, depends on such time of the line having the worst line characteristic.